Economically viable methods for extracting oil from oil shale have been under investigation for many years. Basically, all methods require the oil shale to be heated to a point at which kerogen, a hydrocarbonaceous component of oil shale, decomposes to yield shale oil. Generally, this phenomenon, known as pyrolysis, occurs at temperatures of about 850.degree. F. to 900.degree. F.
Although there are various types of retorts in which pyrolysis can be achieved, this invention is concerned with vertical retorts. In a vertical retorting operation, oil shale in particulate form is introduced through the top of a generally cylindrically shaped vertical retort in which the shale is heated to its pyrolysis temperature. At this temperature the kerogen releases shale oil vapors which are delivered to cooling and oil separating stations to produce both product oil and product gas.
The oil shale in a vertical retort process is heated by either steam or recycled product gas. The use of recycled product gas is convenient because the gas is available from the system and can readily be heated to the required temperature. For reasons not fully known, however, retorting with recycled product gas does not provide the same shale oil yields as when retorting with steam. In tests run to determine the difference in yield between the two systems, retorting yields using recycled product gas as the oil shale heating medium were about 10% less than retorting yields using steam as the heating medium.
The use of steam as the heating medium, even though it causes increased shale oil yields, raises other problems not associated with recycled product gas. The use of steam is more costly than the use of recycled product gas because of high compression costs to allow condensing of steam from the retort off-gas while boiling condensed steam, separated from oil and gases, for recycle. The use of pure steam also has the disadvantage of operating at high pressures, which add to the cost of the retort shell and complicate the lock-hopper design. Further, the site of an oil shale processing facility will generally be close to the area where the oil shale is found. Since such locations typically are found in arid areas the availability and cost of an adequate supply of water, even though recycled water may be used, can be a consideration. The decision as to which gaseous heating medium to use is thus frequently made on the basis of factors other than a determination of which system is capable of the highest yields of shale oil.
From the foregoing, it can be seen that it would be highly desirable to be able to use a heating medium in a vertical retorting operation which is readily available, economically satisfactory and at the same time capable of producing yields comparable to those produced by the use of steam.